In, for example, Japanese Patent Publication No. 3-7039 for example, a housing is provided with a suction port and a discharge port , with a rotor being vertically disposed in the housing and rotatably supported by a pair of ball bearings, with a centrifugal compression stage and a circumferential flow pump stage (or a peripheral flow pump stage) being successively disposed in a portion of the housing between the suction port and the discharge port.
The centrifugal compression stage has a centrifugal impeller carried by the rotor and a centrifugal stator which stationarily surrounds the impeller. The circumferential flow pump stage has a circumferential flow impeller (or peripheral flow pump stage) secured to the rotor and a circumferential stator (or a peripheral flow stator) which stationarily surrounds the impeller.
These pump stages overhang so as to project axially outwardly beyond the ball bearings supporting the rotor. In other words, the end of the rotor adjacent to the suction port is not supported. These pump stages are driven by a motor coupled to the rotor.
This proposed pump offers an advantage in that it can be manufactured at a low cost due to simplicity of the construction but, suffers from disadvantages in that no means are provided for canceling a large thrust force generated due to the hydrodynamic force of the discharged gas and in that high operation speed of the pump is difficult to realize due to too large length of the overhang or unsupported portion of the rotary portion.